Heat-sensitive recording materials, which are in wide practical use, record color images taking advantage of a heat-induced color development reaction between a usually colorless or lightly colored leuco dye and a phenol or an organic acid. Such heat-sensitive recording materials have advantages in that, for example, color images can be formed simply by the application of heat, and further, recording devices therefor can be relatively compact, easily maintained, and noise-free. For this reason, heat-sensitive recording materials have been used in a broad range of technical fields as information-recording materials for facsimile machines, output devices for computers, printing devices such as label printers, automatic ticket vending machines, CD/ATMs, order form output devices for use in family restaurants, data output devices in apparatuses for scientific research, etc.
As the application field for heat-sensitive recording materials continues to expand, there is an increasing likelihood that heat-sensitive recording materials will come into contact with cosmetic creams, oils, or polymer sheets containing plasticizers, or that they will be exposed to severe conditions such as high temperatures and humidity. Such situations also increase the likelihood of problems. For example, with respect to heat-sensitive recording materials obtained by using 2,2-bis(4-hydroxyphenyl)propane (i.e., bisphenol A), p-hydroxybenzyl benzoate, etc., which are conventionally used as developers, when the materials are exposed to high temperatures and/or high humidity, the image density is degraded. When such a heat-sensitive recording material comes into contact with a plasticizer, oil, etc., the image often fades to an extent that it becomes unreadable. Further, under severe conditions such as high temperatures and humidity, unprinted areas become colored, and so-called grayish occurs.
Various hydroxydiphenylsulfone derivatives have been developed and used as developers that are capable of solving these problems. It is known that 4-hydroxy-4′-isopropoxydiphenylsulfone in particular has advantages in that the brightness and sensitivity of obtained heat-sensitive recording layers are high, the preservative properties of obtained recorded images are excellent, etc.
In general, the ingredients of heat-sensitive recording materials, such as leuco dyes, developers and sensitizers, are made into particles by wet grinding before use. In recent years, with an increasing demand for higher sensitivity in heat-sensitive recording materials, the micronization of these particles has been pursued. The microparticulation of 4-hydroxy-4′-isopropoxydiphenylsulfone, however, sometimes causes crystal growth during wet grinding for microparticulation or during the storage of the prepared dispersion. Crystal growth tends to occur especially when 4-hydroxy-4′-isopropoxydiphenylsulfone is ground into microparticles of 1 μm or less. Such crystal growth is attributed to the hydration of 4-hydroxy-4′-isopropoxydiphenylsulfone, the grown crystals being hydrates. Heat-sensitive recording materials produced using a dispersion that contains such a hydrate often suffer from fogging.
According to known methods, in order to prevent hydration of 4-hydroxy-4′-isopropoxydiphenylsulfone, dispersions may be prepared using a specific amount of an alkylated bis(4-hydroxyphenyl)sulfone compound (see patent document 1), using a compound selected from the group consisting of bis(4-hydroxyphenyl)sulfone and metallic salts thereof (see patent document 2), using a specific amount of 4,4′-diisopropoxydiphenylsulfone (see patent document 3), or using a specific amount of diphenyl sulfone derivatives of a specific chemical formula (see patent document 4).
Likewise, according to other known methods, in order to prevent hydration, hydroxypropylcellulose may be used during dispersion (see patent document 5), at least one compound selected from the group consisting of metylcellulose and hydroxyethyl methylcellulose may be used during dispersion (see patent document 6), or hydroxypropyl methylcellulose may be used (see patent document 7). However, there are problems in that, for example, the saturation concentration and sensitivity of such a heat-sensitive recording material upon color development are lowered, and much time is consumed before it is dispersed to a desired particle diameter, causing reduced dispersion efficiency.
Also known is a heat-sensitive recording material containing a dispersion obtained by simultaneously wet dispersing 4-hydroxy-4′-isopropoxydiphenylsulfone and a thermoplastic material having a melting point of 130° C. or less, using a sulfone-modified polyvinyl alcohol as a dispersant (see patent document 8).
According to patent document 8, the obtained heat-sensitive recording material is excellent especially in recording sensitivity, and it resists grayish. However, a thermoplastic material having a melting point of 130° C. or less, such as 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, or 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanuric acid, is used for the purpose of improving recording sensitivity, and its blending proportion to 4-hydroxy-4′-isopropoxydiphenylsulfone is so large that the preferable amount is 30 to 200 wt %. Although patent document 8 discloses that sulfone-modified polyvinyl alcohol can prevent the hydration of 4-hydroxy-4′-isopropoxydiphenylsulfone during wet grinding, sufficient measures to prevent fogging have not yet been achieved.
Heat-sensitive recording materials are sometimes exposed to relatively high temperatures while being stored, while being set in a device, or after printing. Grayish under these conditions must be avoided. The density of an unprinted area after contact with a 70° C. iron plate for 5 seconds is usable as a scale for grayish. This is defined as the property of static sensitivity. It is preferable that such a value be as small as possible. Practically, a value of 0.2 or less is preferable.    Patent document 1: Japanese Unexamined Patent Publication 1996-324126    Patent document 2: Japanese Unexamined Patent Publication 1990-76779    Patent document 3: Japanese Unexamined Patent Publication 1995-314902    Patent document 4: W095/018018    Patent document 5: Japanese Unexamined Patent Publication 1993-162455    Patent document 6: Japanese Unexamined Patent Publication 1993-309951    Patent document 7: Japanese Unexamined Patent Publication 1989-237191    Patent document 8: Japanese Unexamined Patent Publication 1997-207440